Note: This is not necessarily a 'problem' to be fixed but it could be better documented at least.

In pairwise_distances, check_pairwise_arrays is not run if metric is a callable or a metric in PAIRWISE_DISTANCE_FUNCTIONS.
check_pairwise_arrays notably ensures the inputs are 2D and performs check_array, which does more than I fully understand, so I can't confidently comment on whether it is 'needed'.

Most PAIRWISE_DISTANCE_FUNCTIONS metrics run check_pairwise_arrays themselves but the following do not:

• haversine_distances (this is implemented in Cython but does seem to check that X is 2D)
• cosine_distances

For user provided metric callables, I think we leave it up to the user to ensure input is correct. I found this comment (#27456 (comment)) in and old PR, about leaving it to the user to ensure the input is 2D.

We also do have the note:

I am not sure this comment is in the best place, but that is an easy fix. check_array is also not performed for user provided metric callables, but maybe this is not 'needed' ?

cc @jeremiedbb and maybe @adrinjalali ?

Context: noticed while working on #29822